1. Field of the Invention
The present invention relates generally to the field of storage or process vessels for potentially explosive materials and more particularly to a new and improved explosion venting arrangement.
2. Background of the Invention
An internal explosion in a chemical storage or process vessel resulting from the ignition of flammable gas mixtures invariably leads to rapid and high pressure increases. The commonly used means of protection against the damaging effects of an explosion in a chemical reactor vessel or storage silo is an appropriately sized relief valve or vent designed to open at a predetermined pressure. Venting allows the burned and unburned gas to flow out of the vessel to thereby mitigate the pressure rise and decrease the amount of energy available for combustion inside the vessel. In actual practice the pressure drops caused by the outflow of materials through the vent does not compensate for the build-up of pressure from combustions as much as would be expected from measured rates of pressure rise in a closed reaction vessel.
Flame fronts are the most usual way for combustion to proceed. When an accidental self-ignition occurs locally inside a hot spot surrounded by a cold reactive mixture, a combustion wave is initiated from the exothermic center. When the combustion wave takes the form of a detonation characterized by super-sonic propagation speeds in the mixture, venting is useless since vents are unable to react in a sufficiently short time to relieve the pressure. For mild accidental ignition venting is useful since a deflagration is initiated and sub-sonic combustion wave propagation speeds are produced. Propagation is by the diffusive transport of mass and energy and is usually less than several meters per second. In enclosed vessels the rate of pressure rise can usually be detected at the walls of the vessel well in advance of the arrival of the deflagration flame front. Thus, there is usually time for vents to open and affect the course of the explosion.
If a gas mixture is ignited centrally in a sealed, compact vessel of low length-to-diameter ratio, a smooth spherical (laminar) flame propagates outwardly with no significant distortion. Some flame wrinkling may appear during the later stage of flame development, owing to diffusion and/or buoyancy effects, but it usually has no significant effect on the flame growth and, therefore, on the vessel pressure-time history. Accordingly, predictions based upon smooth flame models are able to match the experimental measurements of pressure in closed vessel explosions as documented by D. Bradley and A. Mitcheson, "Mathematical Solutions for Explosions in Spherical Vessels", Combustion and Flame, 26, 201-217 (1976).
The laminar flame model breaks down, however, with vented vessels subsequent to vent opening. The sudden opening of a vent produces a rapid increase in gas-velocity which in turn produces a centered expansion wave or rarefaction wave at the vent. This rarefaction wave propagates back into the vessel and interacts with the expanding flame front. The resulting pressure and velocity disturbance is believed to initiate a hydrodynamic instability within the flame front accompanied by an increase in the combustion rate and rapid pressure development. Thus in a vessel with an initially closed vent, the pressure at first follows the dependency observed for confined expanding flames. There is a slight decline in the pressure-time profile immediately after the vent opens due to the venting temporarily overcoming the combustion rate. Very quickly, however, due to the interaction of the rarefaction wave, the pressure in the vented vessel turns around and rises at a rate considerably greater than that measured in an unvented vessel. This phenomenon limits the accuracy with which proper vent sizing may be determined and as a result generally requires oversize and expensive venting devices and systems larger than might otherwise be indicated.